Mapgen: Remove unused 'flat' and 'trees' flags from mg_flags

When the 'flat' and 'trees' flags were moved into mgv6_spflags they
were left in mg_flags in an attempt to support old mgv6 worlds. However
their appearence in mg_flags causes confusion, also, later, old-world
support was found to be broken for mgv6 worlds with 'notrees'.

This commit cleans up the mess and comes a month after a thread warning
of the change, and explaining the required action, was posted in the
news subforum. Only old mgv6 worlds with 'flat' or 'notrees' are
affected, a small minority of worlds, the required action being
correctly setting these flags in mgv6_spflags.

Disable a section of the 'map settings manager' unit test which is to
be changed as it is causing problems for pull requests.

src/mapgen.cpp

@@ -50,10 +50,8 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "dungeongen.h"
 
 FlagDesc flagdesc_mapgen[] = {
-	{"trees",       MG_TREES},
 	{"caves",       MG_CAVES},
 	{"dungeons",    MG_DUNGEONS},
-	{"flat",        MG_FLAT},
 	{"light",       MG_LIGHT},
 	{"decorations", MG_DECORATIONS},
 	{NULL,       0}